In recent years, a technology known as link aggregation (LAG) is prevalent, in which a plurality of physical lines are collectively treated as a single virtual line. For example, in the LAG technology, N number of physical lines each having the communication band of 100 Mbps are bundled together and treated as a virtual line having the communication band of 100 Mbps×N.
The use of LAG allows for a flexible and efficient network construction. For example, even in case a trouble occurs in one of the physical lines bundled in a virtual line, the communication can be carried on without interruption by making use of the other physical lines that are working without trouble. Besides, the LAG technology is not limited for the processing of a single type of service and can be implemented in order to collectively process various types of services such as VLAN over Ethernet (registered trademark) (VLAN stands for virtual LAN), IP over Ethernet (registered trademark) (IP stands for Internet protocol), and MPLS over Ethernet (registered trademark) (MPLS stands for multi protocol label switching). In other words, the LAG technology enables transmission of frames having different types of services through a single virtual line.
However, in case the frames happen to be concentrated at only some of the physical lines bundled using the LAG technology, then the remaining physical lines remain unused and the communication band constructed using the LAG technology is not used in an effective manner. Conventionally, in order to prevent concentration of frames at only some physical lines, the frames are divided among all of the physical lines.
Explained below is a conventional network configuration in which the LAG technology is implemented. FIG. 11 is an exemplary configuration diagram of a conventional network. As illustrated in FIG. 11, the network includes user terminals 10a to 10h and communication apparatuses 20a to 20f. In the following explanation, the user terminals 10a to 10h are representatively referred to as a user terminal 10; and the communication apparatuses 20a to 20f are representatively referred to as a communication apparatus 20.
The user terminal 10 transmits frames to various destinations via the communication apparatuses 20 and receives frames transmitted by the other user terminals 10 via the communication apparatuses 20. The communication apparatus 20 receives frames transmitted by the user terminals 10 and, based on the address information and the like stored in each frame, transfers the frames to the corresponding destinations. For example, when the user terminal 10 transmits a frame, the communication apparatus 20 performs frame transfer based on a media access control (MAC) address or an IP address stored in the frame.
Explained below is a configuration of the conventional communication apparatus 20 illustrated in FIG. 11. FIG. 12 is a schematic diagram of a configuration of the conventional communication apparatus 20. As illustrated in FIG. 12, the communication apparatus 20 includes line interface (I/F) units 21 to 24, a switch (SW) unit 25, and a controller 26 that is connected to a control terminal 27.
The line I/F units 21 to 24 are connected to a plurality of physical line ports and function as interfaces to external devices for performing frame transmission processing. The SW unit 25 is connected to each of the line I/F units 21 to 24 and provides a switch function for frame transmission between the line I/F units 21 to 24. The SW unit 25 and the line I/F units 21 to 24 transmit/receive data signals of frames or the like.
The controller 26 performs transmission/reception of control signals with the line I/F units 21 to 24 and the SW unit 25 and performs various settings for the line I/F units 21 to 24 and the SW unit 25. Besides, the controller 26 is connected to the control terminal 27 and reports, to the control terminal 27, the information obtained from the line I/F units 21 to 24 and the SW unit 25. Herein, the information obtained from the line I/F units 21 to 24 and the SW unit 25 includes, for example, the information regarding whether a trouble has occurred.
Generally, the line I/F units 21 to 24, the SW unit 25, and the controller 26 illustrated in FIG. 12 are provided in the form of detachable modules or cards. Alternatively, the line I/F units 21 to 24, the SW unit 25, and the controller 26 can also be integrated with the motherboard of the communication apparatus 20.
Explained below is a configuration in which link aggregations are constructed between the communication apparatuses 20 illustrated in FIG. 11. FIG. 13 is a configuration example of communication apparatuses having link aggregations constructed therebetween. In the example illustrated in FIG. 13, link aggregations are constructed between the communication apparatuses 20a and the 20b. 
In FIG. 13, line I/F units 21a to 24a and an SW unit 25a disposed in the communication apparatus 20a respectively correspond to the line I/F units 21 to 24 and the SW unit 25 described with reference to FIG. 12. Similarly, line I/F units 21b to 24b and an SW unit 25b disposed in the communication apparatus 20b also respectively correspond to the line I/F units 21 to 24 and the SW unit 25 described with reference to FIG. 12. The line I/F units 23a and 21b include physical line ports #0a to #7a, while the line I/F units 24a and 22b include physical line ports #0b to #7b. 
With reference to FIG. 13, the physical lines connected to the physical line ports #0a to #7a are bundled to construct a single virtual line; while the physical lines connected to the physical line ports #0b to #7b are bundled to construct a single virtual line. The physical lines connected to the physical line ports #0a to #7a are collectively referred to as LAG #A, while physical lines connected to the physical line ports #0b to #7b are collectively referred to as LAG #B. The paths in the LAG #A are referred to as work paths, while the paths in the LAG #B are referred to as protection paths. The work paths and the protection paths sometimes go through other nodes. In the example illustrated in FIG. 13, the protection paths connect the communication apparatuses 20a and 20b via a different node 5.
The communication apparatuses 20a and 20b make use of the work paths as active paths and keep the protection paths as backup paths in order to ensure redundancy. That is, when the work paths are functioning normally, the communication apparatuses 20a and 20b transfer frames using the work paths. However, in case the work paths encounter a trouble, the communication apparatuses 20a and 20b change over from the work paths to the protection paths for transferring frames without interruption.
Explained below are flows of frames in a case when the user terminal 10a transfers the frames to the user terminal 10c. Firstly, when the line I/F unit 21a receives a frame from the user terminal 10a, it selects a physical line port for transferring the received frame. More particularly, the line I/F unit 21a calculates a hash value based on a hash rule set in the LAG #A being used and selects a physical line port corresponding to the hash value from among the physical line ports #0a to #7a. 
If the physical line port #0a is selected by the line I/F unit 21a, the frame reaches the communication apparatus 20b via the SW unit 25a, the line I/F unit 23a, and the physical line port #0a and then reaches the user terminal 10c via the line I/F unit 21b, the SW unit 25b, and the line I/F unit 23b. 
Explained below is a configuration of the line I/F unit 21a illustrated in FIG. 13. FIG. 14 is a schematic diagram of a configuration of a conventional line I/F unit. As illustrated in FIG. 14, the line I/F unit 21a includes a flow identifying unit 30, an LAG control table 31, an LAG hash calculator 32, a transfer destination controller 33, and a central processing unit (CPU) 34. To the CPU 34 is connected a CPU card 35 and to the CPU card 35 is connected an external monitor 40.
regarding a frame obtained from outside, the flow identifying unit 30 outputs that frame to the transfer destination controller 33. The LAG control table 31 is used to store the hash rule set with respect to each LAG. FIG. 15 illustrates an exemplary data configuration of the LAG control table 31. As illustrated in FIG. 15, in the LAG control table 31, LAG types and hash rules are stored in a corresponding manner. Herein, the LAG types represent the information for identifying link aggregations and the hash rules represent the information that specifies the information used at the time of calculating hash values. For example, in the case of transferring a frame using the LAG #A, hash calculation is performed using the MAC address in that frame.
The LAG hash calculator 32 obtains, from the transfer destination controller 33, hash calculation target information and performs hash calculation based on that information. Herein, the hash calculation target information represents, for example, MAC addresses or IP addresses. The LAG hash calculator 32 then outputs the hash values obtained as the calculation result to the transfer destination controller 33. With respect to identical hash calculation target information, the LAG hash calculator 32 calculates the same hash value.
The transfer destination controller 33 makes use of the LAG control table 31 and the LAG hash calculator 32 and determines physical line ports to be used for frame transfer.
More particularly, the transfer destination controller 33 first determines the hash rules by comparing the LAG types to be used in frame transfer with the contents of the LAG control table 31. Subsequently, based on the determined hash rules, the transfer destination controller 33 outputs the hash calculation target information from the frames to the LAG hash calculator 32 and then obtains respective hash values from the LAG hash calculator 32.
For example, if the LAG #A is used for frame transfer, then the hash rule is “MAC address”. In that case, the transfer destination controller 33 outputs the MAC addresses of frames to the LAG hash calculator 32 and then obtains respective hash values from the LAG hash calculator 32.
Meanwhile, the transfer destination controller 33 includes a selection table in which the hash values and the physical line ports are stored in a corresponding manner. Thus, the transfer destination controller 33 compares the hash values obtained from the LAG hash calculator 32 with the hash values in the selection table and determines the physical line ports to be used for frame transfer.
The CPU 34 obtains various control signals from the CPU card 35 and accordingly performs settings of the flow identifying unit 30, the LAG control table 31, the LAG hash calculator 32, and the transfer destination controller 33.
Thus, the CPU card 35 outputs various control signals to the CPU 34 so that the CPU 34 can perform settings of the flow identifying unit 30, the LAG control table 31, the LAG hash calculator 32, and the transfer destination controller 33. Besides, the CPU card 35 outputs information such as the setting results to the external monitor 40. As to an example of conventional technology, see Japanese Laid-open Patent Publication No. 2007-180891.
However, in the abovementioned conventional technology, there are times when only some of the physical line ports are used as frame destinations in a concentrated manner. In such cases, even if a plurality of physical lines is present, each physical line is not put to use in an effective manner.
The problem of concentrated use of only some physical line ports as frame destinations arises if the hash calculation target information is identical in a plurality of frames. In that case, the respective hash values obtained as the calculation result are also identical. What causes the abovementioned problem is that the hash values and the physical line ports correspond on one-on-one level.
For example, consider the case of transferring a plurality of frames having different types of services. In that case, if the destination for all frames is identical, then the MAC addresses/IP addresses in those frames are the same. Moreover, if the LAG hash rule is assumed to be the MAC address or the IP address, then the hash value of each frame becomes identical. Because of that, the frames happen to be concentrated at a particular physical line.